Answer
There is no work done by a net external force on the boat.
Work Step by Step
The work-energy theorem states that $$W=\frac{1}{2}m(v_f^2-v_0^2)$$ $W$ is the work done by a net external force on the object
If the sailboat moves at a constant velocity, $v_f^2-v_0^2=0$, so $W=0$. Therefore, there is no work done by a net external force on the boat.
